The expressions may contain any number of variables. To make the computation, you need to invoke the element evaluate() function with a Javscript object with all these variables as properties.

For instance, if your function element is called myFunction and the expression it contains is x+y, you can evaluate the function as in:
var result = _view.myFunction.evaluate( { x : 3 , y : 2 } );
result should be 5.

If your expression contains many parameters (variables which rarely change their value), as in a*x^3+b*x^2+c*x+d, you can save the effort of passing each time the values of the parameters a,b,c,d, by setting:
_view.myFunction.setParameters( { a : 1 , b : 3 , c : -1, d : -2 } );
and then evaluate the function by simply invoking:
var result = _view.myFunction.evaluate( { x : 3 } );
The parameters do not change until you call setParameter again.

If you invoke once:
var result = _view.myFunction.evaluate( { a : 0, x : 3 } );
then this value of a will be called for this particular evaluation. But the next time, the value of the parameter a will still be 1.